Figure 1

Reaction scheme for the homotropic two-state model, HOTSM. R is a receptive entity and S is a ligand, that can bind to two sterically separate sites in R. This is signified by positioning at either side of symbol R. The receptive unit in the unliganded condition exists in two conformations, a reactive form R and an active form R*. Arrows pointing into the plane of paper indicate the flow direction of signal transduction. There are 7 equilibrium constants in the HOTSM reaction scheme. All symbols are explained in the text.